Displaying tangents as colors
My bad, tangents successfully appears in shader when I switched viewport shading to "Material". 

Testing code: 

import bge

cont = bge.logic.getCurrentController()

VertexShader = """
    attribute vec4 tangent;
    varying vec4 frag_tangent;
    void main()
    {
        frag_tangent = tangent;
        gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
    }
"""

FragmentShader = """
    varying vec4 frag_tangent;
    void main()
    {
        gl_FragColor = frag_tangent;
    }
"""

mesh = cont.owner.meshes[0]
for mat in mesh.materials:
    shader = mat.getShader()
    if shader != None:
        if not shader.isValid():
            shader.setSource(VertexShader, FragmentShader, 1)
shader.setAttrib(bge.logic.SHD_TANGENT)


It works great when viewport is in right shading mode. 

Thanks :)